1. Technical Field
The present invention generally relates to graphic image manipulations and in particular to manipulation of map images. Still more particularly, the present invention relates to the coordinating the manipulation of multiple map images displayed on a data processing system.
2. Description of the Related Art
Modern geographic information systems normally make use of digital vector-based map information. However, a vast legacy of paper-based map information exists. It is very expensive and time consuming to convert all of the information on these paper maps over to a digital vector format. In many cases the scope and expense of such conversions renders them completely impractical. However, even when a complete conversion to digital vector-based format is not possible, it is still possible to obtain some of the benefits of computerized map systems, first by converting the paper maps to digital raster maps by digitally scanning them, and then by georeferencing the raster image.
A digital map image is said to be georeferenced if a pair of mathematical functions, f, and g, have been determined that can be used to convert back and forth between the coordinates of the map image (as defined by the pixels of the image) and the corresponding longitude and latitude of the location of that point. That is, f and g do the following:
1. If (x, y) represents a location on the digital map image, then f (x, y)=(Lon, Lat) represents the longitude and latitude of the corresponding physical location.
2. If (Lon, Lat) represents a physical location that lies within the region covered by the map, then g(Lon, Lat)=(x, y) represents the point on the digital map image that corresponds to that longitude and latitude.
Here, x and y represent the natural internal coordinate system of the map image. In most cases, a vector-based map image uses longitude and latitude as its internal coordinate system, if so, it can be considered to be trivially georeferenced already.
Typically a digital raster map image uses the pixels of its image as a kind of natural coordinate matrix. This type raster map image will require non-trivial georeferencing functions to convert back and forth between coordinate systems.
In a geographic information system, both raster maps and vector maps are often used, since raster maps can be easily obtained from the vast wealth of paper maps available, and vector maps can contain a great amount of underlying data. When each of these maps are displayed, users will typically desire to manipulate the view, by scrolling, zooming, or otherwise. If more than one map is being displayed, the user is typically required to independently manipulate each map to the desired view. It would be desirable to provide a means for a user to simultaneously manipulate both maps, even when the maps use different internal coordinate systems.